The term "molecular sieve" is applied to certain compositions because of their most useful property--the ability to separate mixtures of compounds with which they are contacted. The better known natural zeolites which were first used for such purposes include chabazite, faujasite, and analcite. Chabazite is a natural zeolite having the ability, for example, to adsorb unbranched hydrocarbons while rejecting branched hydrocarbons and, hence, is referred to as a molecular sieve.
The classical molecular sieve, whether natural or synthetic, exhibits certain properties which are undesirable for many purposes. For example, a zeolitic molecular sieve is not chemically inert, it has polar surface characteristics (i.e., is hydrophilic), may be adversely affected by high temperature, and may be comparatively expensive. Insofar as the properties of a molecular sieve are determined by physical adsorption, the term "sieve" is misleading, since there is no separation of "rejected" and "passed" materials as in a screen or sieve; one or more components of the mixture treated is adsorbed and the rest are not adsorbed.
Granular activated carbon is normally thought of as having a mesh size in a range of, perhaps, 4 .times. 10 to 20 .times. 50 U.S. Sieve Series. It will have a surface area (determined by the adsorption of N.sub.2) of at least 600 square meters per gran but generally not more than about 1,300 square meters per gram and may adsorb an amount of carbon tetrachloride equivalent to about 50 percent of its weight. The surface area is roughly a measure of the capacity of the carbon and is conventionally determined by the quantity of iodine adsorbed. A discussion of a commercial activated carbon and its characteristics may be found in Zabor's U.S. Pat. No. 2,763,580.
Prior to the present invention, activated carbons had been prepared which exhibited molecularly selective properties not usually observed. See, for example, Walker, P. L.; Lamond, T. G.; and Metcalfe, J. E., 2d Conf. on Industrial Carbon and Graphite (1965). The authors used commercial activated carbons, coated them with significant amount of certain resins, and carbonized them at temperatures ranging from 500.degree.C to 850.degree.C. Comparisons were then made with commercial synthetic zeolitic molecular sieves. See, also, Evans and Flood, U.S. Pat. No 3,516,791 and Konrad, U.S. Pat. No. 2,790,511.
M. M. Dubinin et al, reporting in Nature, V. 207, P. 75-76 (July 3, 1965), prepared an activated carbon from wood which exhibited pore size properties comparable to zeolitic or synthetic molecular sieves but which had very low surface areas. J. E. Metcalfe III, M. Kawahata, and P. L. Walker, Jr., applied the term "molecular sieve" to certain activated anthracite coals, indicated to have a low surface area. [Fuel, 42, 233 (1963)].
W. F. K. Wynne-Jones, in the proceedings of the Tenth Symposium of the Colston Research Society, P. 35 (1958), studied the surface area of activated carbon and made comparisons to commercial zeolitic synthetic molecular sieves.
Activated carbons made from such source material as polyvinylidene chloride, polyvinylbenzene, urea-formaldehyde resins, etc., have exhibited properties which have not found general use. See for example, J. J. Kipliing and R. B. Watson, "Adsorptive Properties of Polymer Carbons," Trans Faraday Society 56, 557, 562 (1960); J. R. Dacey and D. G. Thomas "Adsorption on Saran Charcoal" Trans Faraday Society 50, 647, 740 (1954); Mahajan and Walker, "Krypton Adsorption on Microporous Carbons and 5A Zeolite," J. Colloid and Interface Science, V. 29, No. 1, P. 129 (1969).